Inorganic pigments are used as opacifiers and colorants in many industries including the coatings, plastics, and paper industries. In general, the effectiveness of the pigment in such applications depends on how evenly the pigment can be dispersed. For this reason, pigments are generally handled in the form of a finely divided powder. For example, titanium dioxide, the most widely used white pigment in commerce today due to its ability to confer high opacity when formulated into end-use products, is generally handled in the form of a finely divided powder. However, titanium dioxide powders are inherently dusty and frequently exhibit poor powder flow characteristics, especially during formulation, compounding, and manufacture of end-use products. While free-flowing powders with low dust properties can be obtained through known manufacturing practices, these powders usually exhibit reduced opacifying properties.
To this end, chemical methods of modification of titanium dioxide pigment surfaces have been developed to achieve the desired balance of pigment opacity and flow characteristics. For instance, it is known in the art that the wetting and dispersing properties of titanium dioxide pigments can be improved by the deposition of inorganic metal oxides and/or metal hydroxides onto the surface of a titanium dioxide intermediate, such as produced, for example, by the vapor phase oxidation of titanium tetrachloride. Processes and apparatus for the vapor phase oxidation of titanium tetrachloride are well-known, see, for example, U.S. Pat. Nos. 3,208,866, 3,512,219, 5,840,112, 6,207,131 and 6,350,427, wherein typically the titanium dioxide intermediate is cooled immediately upon leaving the reactor to yield a solid, agglomerated material which is then further processed in order to provide a finished product. Conventionally the further processing of the agglomerated intermediate involves the following:
(1) dispersing the intermediate (or crude) material in an aqueous medium using a dispersing agent such as a polyphosphate,
(2) wet milling the resulting slurry,
(3) precipitating one or more inorganic oxides, for example silica and/or alumina, onto the particle surfaces of the wet milled titanium dioxide slurry,
(4) recovering the inorganic oxide-treated titanium dioxide pigment from the aqueous slurry by filtration,
(5) washing the filtered product to remove residual salts and impurities,
(6) drying the washed filtered product, and
(7) dry-milling the dried pigment using a fluid energy mill.
The deposition of inorganic oxides according to step (3) provides some desired pigment end-use properties, and typically also provides improvements in the further processing of the pigment. For example, silica is typically added to impart improved resistance to the deleterious effects of ultraviolet light in pigmented end-use applications, whereas alumina is typically added to ensure smooth processing through filtration, drying, and fluid energy milling, as well as to impart improved dispersibility characteristics to the finished pigment in end-use applications.
It is also known in the art to chemically treat pigment intermediates, and in particular titanium dioxide, with zirconium compounds, in order to further impart improved gloss and durability properties to the final pigment product. Given the high efficiency with which zirconium compounds impart their known benefits, it is not surprising that many patents have been issued describing methods for improving pigments, including titanium dioxide, wherein zirconia is deposited onto the pigment surface.
U.S. Pat. No. 4,405,376, for example, discloses a titanium dioxide pigment along with a process for producing the pigment, where the pigment displays improved durability and dispersibility and comprises a pigmentary titanium dioxide core particle, an inner coating of hydrous oxides of tin and zirconium, and an outer coating of hydrous oxide of aluminum.
U.S. Pat. No. 4,450,012 discloses coated mixed phase rutile pigments having a first coating of an oxide or hydrated oxide of titanium, zirconium, tin or a mixture of these and a subsequent coating of an oxide or hydroxide of aluminum, which pigments resist flocculation when formulated in lacquers hardened with acid catalysts.
U.S. Pat. No. 4,640,716 claims a pigment comprising a zirconium-treated uncalcined kaolin clay, made by combining the clay with an added source of zirconium ion under conditions including the absence of deleterious amounts of a binder and the presence of at least a sufficient amount of the zirconium ion source to enhance the optical character of a substrate containing the dry pigment. The preferred zirconium ion source is ammonium zirconium carbonate. The pigments are useful in providing smooth, opaque surface finishes on fibrous web substrates such as high quality printing papers.
U.S. Pat. No. 4,759,800 teaches a process for the improvement of zirconia treated titanium dioxide pigments by a post-treatment wherein the titanium dioxide pigment is treated with an aqueous titanium oxide chloride solution, and optionally, other oxide-forming water-soluble metal salts. The pigments so produced are said to exhibit improved weather resistance and optical properties.
U.S. Pat. No. 5,203,916 describes a pigmentary titanium composite possessing good durability and excellent optical properties, consisting essentially of a particulate titanium dioxide base, a hydrous zirconium oxide layer deposited on the titanium dioxide base, and a hydrous alumina layer deposited on the hydrous zirconium oxide layer. The invention also claims the process for producing such a pigment.
U.S. Pat. No. 5,755,870 describes titanium dioxide-based composite aggregate pigments, also described as chemically aggregated extender pigment composites, produced through the aggregation of titanium dioxide and calcined clay with zirconium hydroxy complex chemicals in a slurry having a pH range of 6.0 to 10.5. Superior optical properties are attributed to the pigments, after the pigments are removed from the slurry and heat treated.
U.S. Pat. No. 5,814,143 discloses synthetic alkali metal silicate pigments surface-modified with oxy-hydroxy zirconium species, and methods for their manufacture. These compositions are said to provide improvements over conventional synthetic alkali metal silicate pigments in terms of optical properties, particularly in paper applications, and physical properties, particularly in rubber applications. In addition, these surface-modified pigments are indicated as useful extender or replacement pigments for titanium dioxide or other synthetic alkali metal or clay aluminosilicate pigments.
U.S. Pat. No. 5,846,310 describes spherical silicon dioxide particles with a size of from 5 to 500 nm coated with titanium dioxide, ferric oxide, or zirconium dioxide particles with a size of less than 60 nm. The coated particles can be after-coated with silanes or further metal oxides. The products obtained are used for pigmenting paints, printing inks, plastics, and coatings or as sunscreen agents.
U.S. Pat. No. 5,976,237 discloses durable coated inorganic pigments with good optical properties, dispersibility, and chemical stability in both paint and plastics applications, which pigments comprise a first coating of alumina, silica, or mixtures thereof deposited on said inorganic pigment prior to said pigment's passing through its isoelectric point, an optional coating selected from the group consisting of zirconia, stannic oxide, silica, titania, and cerium oxide, and mixtures thereof deposited over said first coating after said pigment has passed through its isoelectric point; and a final coating of alumina deposited over the first coating and optional coating. The preferred inorganic pigment is titanium dioxide.
U.S. Pat. No. 6,200,375 concerns a surface-treated titanium dioxide pigment for use in high quality exterior paints. The aforementioned surface treatment comprises a layer of zirconium hydroxide or oxyhydroxide, then a layer of titanium hydroxide or oxyhydroxide, then a layer of co-precipitated phosphate and silica, and finally a layer of aluminum oxyhydroxide. These pigments are also described as useful in plastics and paper laminates.
U.S. Pat. No. 6,656,261 B2 describes substantially sulfate-free titanium dioxide pigments with improved gloss and/or durability comprising alumina, zirconia, and optionally, phosphate compounds, and methods of making these pigments. These pigments are useful in the manufacture of paints and plastics. This patent also claims a method comprising wet treating titanium dioxide with first, an alumina compound to form an alumina layer, followed sequentially by wet treatment with a zirconia compound to form a zirconia layer, but nothing is taught as to the expected outcome. The claiming of an outer zirconia layer is in sharp contrast with the other references known to the inventors and describing the use of a zirconia inorganic oxide treatment, wherein the deposition of zirconia has been carried out as an intermediate step only and a final alumina treatment has been applied in order to ensure the pigment's compatibility with modern ingredients used in paints, plastics, and paper, for instance.
U.S. Patent Application 20060032402 relates to a weather resistant titanium dioxide pigment with good optical properties, having a multi-layered dense silicon dioxide skin doped with metal ions selected from the group comprising tin, zirconium, and titanium, wherein at least one of the two or more dense silicon dioxide layers does not contain any significant quantity of metal atoms other than silicon. The pigment is particularly suitable for use in surface coatings and plastics.
U.S. Patent Application 20060034739 relates to a method for the post-treatment of titanium dioxide resulting in weather-resistant titanium dioxide pigments with good optical properties, wherein together with the hydrous oxides of tin and zirconium, at least one other oxide from the group comprising aluminum, silicon, and titanium is additionally precipitated onto the particle surface. The resulting pigment is particularly suitable for use in paints, coatings, and plastics.
In those documents cited above where enough details are provided to fully discern the conditions under which zirconia is deposited onto a pigment surface, and with particular reference to titanium dioxide pigments, the methods disclosed typically describe procedures wherein zirconia is deposited under initially acidic pH conditions, as for instance U.S. Pat. Nos. 4,405,376; 4,450,012; 5,203,916; 6,200,375; 6,656,261, and U.S. Patent Application Publication 20060034739, in order to take full advantage of commercially available salts of zirconium, which are typically soluble in water only under acidic conditions, and usually impart an acid response when dissolved. Thus it is not surprising that zirconia deposition is largely prescribed at low pH. In the few cases where zirconia treatment is taught as being carried out under alkaline pigment surface treatment conditions, for instance U.S. Pat. No. 4,759,800, and U.S. Patent Application Publications 20060032402 and 20060034739, it is not clear that any advantage accrues from the addition of the preferred acidic salts of zirconium into an alkaline treatment environment.
Given the extensive prior art relating to the known benefits imparted by deposited zirconia to titanium dioxide pigments, and despite all the work and effort previously documented relating to the development of improved zirconia treatment processes for titanium dioxide pigments, further improvements are continually being sought, especially given the high raw material cost of zirconium chemicals in general. In none of the aforementioned references are processes for the manufacture of zirconia-treated pigments described which would anticipate the advantages achieved according to the instant invention, details of which are provided below.